In a typical AMOLED display panel, the TFT device circuits are formed on a TFT back panel of the display panel. The TFT devices, which generally include a polycrystalline silicon film as a semiconductor layer, may be a bottom gate type or a top gate type, such as low temperature polysilicon thin film transistor. The polycrystalline silicon film requires high electron mobility in order for the TFT device to function optimally. In general, the polycrystalline silicon film is formed from an amorphous silicon film. One way to form the polycrystalline silicon film from the amorphous silicon film is to crystallize the amorphous silicon film by irradiating it with laser light, such as a high-power excimer laser. An excimer laser is a pulsed laser having KrF, ArF, or XeCl as a light source. The amorphous silicon film is generally crystallized over its entire surface by irradiating the substrate from one end to the other with excimer laser light that has been processed to have a linear shape. The linear shaped laser beam generally spans a portion or the whole length of a TFT back panel and is scanned in a lateral direction.
Illustrated in FIG. 1 is a 4×4 pixel array portion of a conventional AMOLED's TFT back panel 100. As illustrated, pixel region 110 comprises a TFT circuit portion 112 and an OLED circuit portion 114. The amorphous silicon film layer is initially deposited over the entire TFT back panel 100 and crystallized into polycrystalline form using the excimer laser annealing process. A linear-shaped excimer laser beam 120 is scanned over the entire surface of the TFT back panel 100 by irradiating a portion of the TFT back panel 100 at a time. Since the size of the laser beam is limited, many pulses of laser beams are required to cover the entire TFT back panel 100.
After the amorphous silicon film is laser annealed into polycrystalline film, subsequent photolithographic process steps remove unnecessary portions of the polycrystalline film except for the polycrystalline islands that are required for the source, drain and channel regions of the TFT devices in the TFT circuit portion 112. But, as illustrated in FIG. 1, the width WL of the laser beam 120 is wider than the TFT circuit portion 112 and irradiates more than just the TFT circuit portion 112 of the amorphous silicon film covering the TFT back panel 100. For example, the width WL of excimer laser beam 120 commonly used in this application is about 400 micrometers, whereas, the width of the TFT circuit portion 112 is about 100 micrometers. Thus, the laser annealing process crystallizes the amorphous silicon film covering the OLED circuit portion 110 of the TFT back panel 100 as well. Although the polycrystalline silicon film is subsequently removed from the OLED portion 110, this often results in undesirable line mura defects in the finished AMOLED display panel.
Mura defects are defects that exhibit as non-uniform contrast regions on an LCD or an OLED display panel and are attributed to pulse-to-pulse variations in the laser beam energy that is used to crystallize the amorphous silicon film. These defects are more pronounced when a constant gray value image or pattern is displayed. In AMOLED display panels, the laser anneal irradiation of the non-TFT regions, such as the OLED circuit portion 110, on the TFT back panel often results in line-shaped mura defects. The non-uniform laser beam energy caused by pulse-to-pulse variations in the laser beam energy results in non-uniform performance of polycrystalline silicon. And because the TFT characteristic is sensitive to the performance of the polycrystalline silicon and the TFT devices drive the OLED devices, the non-uniform TFT characteristics result in non-uniformity in OLED's brightness. This non-uniformity causes the line mura defects.
To eliminate the line mura defect problem, conventional laser annealing process for crystallizing the amorphous silicon film calls for overlapping each pulse of the laser beam to minimize the effects of the pulse-to-pulse variations in the laser beam energy. Furthermore, the silicon film is scanned with the laser beam twice to further minimize the effects of the pulse-to-pulse variations in the laser beam energy. But these conventional solutions are expensive because the processing time is increased and the life of the laser is shortened because of the increased duty cycle.
Also, because substantial portion of the laser beam energy is spent in irradiating unnecessary portions of the amorphous silicon thin film areas, the conventional AMOLED circuit layout results in an inefficient use of manufacturing resources. And the unnecessary expenditure of the laser beam energy attributes to unnecessarily shortening the life of the excimer laser tool.